This invention relates generally to a hydraulic fluid power circuit, and more particularly to a motor vehicle hydraulic fluid power circuit having a hydraulic pump for supplying fluid pressure to assist the operator in steering and breaking the vehicle.
Motor vehicles are frequently provided with a hydraulic pump which is driven by the engine of the vehicle to provide a source of fluid power. In passenger cars, this fluid power is commonly used to assist the operator in steering the vehicle. Because of increasing automobile braking performance requirements and because of a decreasing ability of intake manifold vacuum braking systems to provide that performance, it has been suggested to use the fluid power of the engine driven hydraulic pump to assist the operator in braking the vehicle, as well as to assist the operator in steering the vehicle.
In such motor vehicle fluid power circuits, it is also desirable to provide one ratio of brake pedal movement to master cylinder movement under normal operating conditions and to provide a different ratio in the event of a fluid pressure failure in the system. This is desirable because one such ratio will provide the brake pedal movement and pedal feel characteristics to which drivers are now accustomed with vacuum actuated braking systems, while a different ratio is required to provide high braking pressures with low pedal effort in the event of pump failure.
In order to provide one ratio during normal operation and a second ratio in the event of pump failure, both mechanical and hydraulic ratio change devices have been proposed. Mechanical ratio change brake boosters are shown in U.S Pat Nos. 3,733,966 and 3,733,968. Hydraulic ratio change brake boosters are shown in U.S. Pat. No. 3,793,829 (which provides a ratio change at the outlet end of the brake booster) and in U.S. Pat. Nos. 3,831,491 and 3,838,629 (which provide a ratio change at the inlet end of the brake booster).